1. Field of the Invention
The present invention relates, in general, to sensors and, in particular, to electrochemical-based sensors.
2. Description of the Related Art
The use of electrochemical-based sensors that employ a redox mediator and a redox enzyme in conjunction with an electrode(s) for the determination of an analyte in a liquid sample has become of heightened interest. Such electrochemical-based sensors are believed to be particularly suitable for continuous or semi-continuous monitoring of analytes (such as glucose) in bodily fluid samples (e.g., blood or interstitial fluid samples). For example, electrochemical-based glucose sensors employing a redox mediator, a redox enzyme and a working electrode can determine (i.e., measure) glucose concentration using relatively low potentials (e.g., less than 0.4 V vs SCE), thereby limiting any interfering responses, at the working electrode. For a further description of electrochemical-based sensors, see for example, U.S. Pat. Nos. 5,089,112 and 6,284,478, each of which is hereby fully incorporated by reference.
In typical electrochemical-based sensors, the redox mediator facilitates electron transfer between the redox enzyme(s) and an electrode(s) of the electrochemical-based sensor. In doing so, the redox enzyme cycles between oxidized and reduced states, driven by the presence of analyte, a redox mediator and a surface of the electrode. The net result of such cycling is that electrons are either accepted or donated at the surface of the electrode while the redox enzyme essentially maintains its original oxidation state and catalytic characteristics.
For the determination of an analyte in an aqueous liquid sample (e.g., a bodily fluid sample such as blood, urine or interstitial fluid), a degree of water solubility for both the redox enzyme and redox mediator can be beneficial in terms of enabling adequately rapid reaction kinetics. Therefore, conventional electrochemical-based sensors may incorporate a redox enzyme and a redox mediator that are solvated in an aqueous liquid sample.
For electrochemical-based sensors that require long term stability, such as continuous or semi-continuous electrochemical-based glucose sensors, it is essential that the redox mediator does not leach away from the vicinity of the electrode. In addition, if the redox mediator is a substance that is harmful to humans or other subjects, leaching of the redox mediator into a human's or other subject's body is undesirable and thus to be avoided.
Redox mediators have been attached to water-insoluble synthetic polymer chains, such as polysilozanes, in order to prevent leaching. Such chemical compositions, however, suffer from low flexibility, and thus reduced reaction kinetics in aqueous media, due to their hydrophobic nature. Moreover, redox mediators covalently attached to hydrophilic polymer backbones are not suitable for efficient and secure conventional attachment directly to an electrode(s) of an electrochemical-based sensor.
Still needed in the field, therefore, is an electrochemical-based sensor that employs a redox enzyme and redox mediator, yet does not suffer from inadvertent leaching of the redox enzyme and/or redox mediator from the vicinity of the electrochemical-based sensor's electrode. In addition, the redox mediator and redox enzyme of such an electrochemical-based sensor should exhibit adequately rapid reaction kinetics.